Small portable computers having a fixed amount of RAM therein are often designed to operate from relatively small and conventional DC power supplies which have to be replaced at increasingly frequent intervals as the computational demands of these computers are increased. These computers normally contain a relatively large amount of RAM therein which is provided to handle the various computational demands of a wide variety of computer software programs. However, in actual practice and average daily use of these computers, only a small fraction of the RAM memory capacity is required by the majority of the computer programs used by a computer operator or at any given point in time such as between the use of applications.
In the past, in has been a common practice to maintain a sufficient DC power level to the RAM to maintain each and every memory storage cell or site therein active and in a data storage and retention mode regardless of the actual cell use. Many recently developed portable computers use a large number of stacked capacitor-type dynamic random access memories (DRAMs) where there is an integrated circuit storage capacitor associated with each memory storage cell and used for storing digital data in the integrated circuit memory chip. Thus, the periodic refresh power required to maintain the necessary stored charge on all of these capacitors and memory cells on the DRAM chip is quite significant when considering the large amount of data storage capacity that is available in these DRAM chips and required for certain types of software applications.
On the other hand, however, most software applications and programs require only a small percentage of the data storage capacity available in the DRAM chip, so that large amounts of power are wasted in refreshing the memory cells on the chip which are not used by these latter, most frequently used software applications. For example, certain types of Macintosh personal computers use up to 32 megabytes of RAM, and the power required to refresh this memory has been estimated to be about 11.2 watts. However, some of the system software used to run these computers uses only approximately 1 megabyte of this RAM, so that when no other applications or programs are running, the power required to maintain and preserve the contents of only the used memory could be reduced to 1/32nd of that previously required, or to 0.35 watts. In this example, a 15 ampere hour battery would last only about one hour with the full 32 megabytes of DRAM being refreshed, while it might last 20 or more hours in a standby mode with only 1 megabyte of DRAM being refreshed.